1. Field of the Invention
This invention relates in general to chip-on-board applications for integrated circuit dice, and in particular to devices and methods for electrically and thermally coupling to the backsides of dice in such applications.
2. State of the Art
Integrated circuit (IC) dice or "chips" are small, generally rectangular IC devices cut from semiconductor wafers, such as silicon wafers, on which multiple IC's have been fabricated. Most dice are packaged by attaching them to lead frames supported in plastic or ceramic packages, and the lead frames and packages are typically designed to conduct heat away from the dice in order to protect them from heat damage. Such packages are also typically designed to protect the dice from corrosion.
Some dice, however, are packaged in what are known as "chip-on-board" (COB) applications, in which the dice are directly attached to printed circuit boards (PCB's) or other known substrates using well-known die-attach techniques. In these applications, bond pads on one or more dice are connected to signal traces on the surfaces of PCB's or other substrates using wire, or tape-automated, bonding, and a liquid or gel encapsulant, commonly referred to as a "glob top," is applied over the dice to protect them from corrosion. One such COB application is described in U.S. Pat. No. 5,497,027.
Dice in COB applications typically generate more heat than their associated PCB's alone can satisfactorily conduct away. Consequently, certain techniques have been devised to assist in conducting heat away from dice in COB applications. In one such technique shown in FIG. 1, circuit traces 10 that widen near a die 12 and are directly attached to the surface 14 of a PCB 16 assist the PCB 16 in conducting significant amounts of heat away from the die 12. Unfortunately, circuit traces that widen sufficiently near dice to satisfactorily conduct heat away from the dice typically use an inordinately large amount of surface space on their associated PCB's. As a result, widened circuit traces can be difficult or impossible to implement in today's densely packed COB applications. Widened circuit traces can also lead to an undesirable increase in capacitive and inductive parasitics, which are highly undesirable for high-speed applications.
Another technique for conducting heat away from dice in COB applications involves using thermally conductive PCB's in place of the more commonly used glass-epoxy PCB's. Thermally conductive PCB's are made with materials such as insulated aluminum, porcelainized steel, and ceramics that are superior in heat transfer characteristics than glass-epoxy. Because this technique is not applicable to the glass-epoxy PCB's used in the majority of COB applications, it is of limited utility.
Therefore, there is a need in the art for a device and method for satisfactorily conducting heat away from dice that are directly attached to a variety of PCT'S, including conventional, widely-utilized glass-epoxy PCB's, without degrading the electrical characteristics of the interconnecting circuitry in COB applications.